U.S. patent application number 09/131694 was filed with the patent office on 2002-04-04 for surface light source device of side light type.
Invention is credited to OHKAWA, SHINGO.
Application Number | 20020039287 09/131694 |
Document ID | / |
Family ID | 16882909 |
Filed Date | 2002-04-04 |
United States Patent
Application |
20020039287 |
Kind Code |
A1 |
OHKAWA, SHINGO |
April 4, 2002 |
SURFACE LIGHT SOURCE DEVICE OF SIDE LIGHT TYPE
Abstract
Illuminating light, supplied from a primary light source of a
surface light source device of side light type, passes through an
incidence surface (12A) and enters a wedge-shaped scattering guide
plate (12). Inside the guide plate (12), the illuminating light
propagates toward the end while being repeatedly reflected between
a back face (12B) and an emission surface (12C). During this
process, the illuminating light receives scattering action of
particles or the like in the guide plate (12). The emission surface
(12C) provides a prism surface. The back face (12B) comprises a
plurality of projections in a belt region (AR1) along an edge (EL).
The projections run generally parallel to the edge (EL) and each
has a pair of slopes (12G and 12H). The height of the projections
(that is, depth of the troughs) or the inclination of the slopes
(12G and 12H) preferably decrease according to distance from the
edge (EL). The projections diffuse light propagation direction,
thereby preventing generation of bright lines around the edge (EU)
of the emission surface.
Inventors: |
OHKAWA, SHINGO;
(KOSHIGAYA-SHI, JP) |
Correspondence
Address: |
JAMES D HALSEY JR
STAAS & HALSEY
700 ELEVENTH STREET NW
SUITE 500
WASHINGTON
DC
20001
|
Family ID: |
16882909 |
Appl. No.: |
09/131694 |
Filed: |
August 10, 1998 |
Current U.S.
Class: |
362/601 ;
362/23.16; 362/330; 362/339 |
Current CPC
Class: |
F21V 5/02 20130101; G02B
6/0061 20130101; G02B 6/0038 20130101 |
Class at
Publication: |
362/31 ; 362/330;
362/339; 362/26 |
International
Class: |
F21V 007/04; F21V
008/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 11, 1997 |
JP |
228853/1997 |
Claims
What is claimed is:
1. A surface light source device of side light type comprising a
primary light source and a guide plate, having an emission surface
and a back face as major surfaces, said guide plate further
comprising an incidence end surface, for receiving light supplied
from said primary light source, and a side face adjoining said
incidence end surface, said back face and said side face meeting so
as to form an edge; wherein a plurality of projections are provided
on said back face in a region located along said edge, said
projections running generally parallel to said edge.
2. A surface light source device of side light type comprising a
primary light source and a guide plate, having an emission surface
and a back face as major surfaces, said guide plate further
comprising an incidence end surface, for receiving light supplied
from said primary light source, and a side face adjoining said
incidence end surface, said back face and said side face meeting so
as to form an edge; wherein a plurality of projections are provided
on said back face in a region located along said edge, said
projections running generally parallel to said edge; and a great
number of projections, running generally perpendicular to said
incidence end surface, are provided on said emission surface.
3. The surface light source device of side light type according to
claims 1 or 2, wherein projections, provided in said region on said
back face, decrease in height as distance from said edge
increases.
4. The surface light source device of side light type according to
claims 1 or 2, wherein projections, provided in said region on said
back face, decrease in sharpness as distance from said edge
increases.
Description
BACKGROUND
[0001] 1. FIELD OF THE INVENTION
[0002] The present invention relates to a surface light source
device of side light type, and more particularly to a technique for
improving quality of illuminating light in the device. The surface
light source device according to the present invention is applied
to, for instance, backlighting of a liquid crystal display.
[0003] 2. RELATED ART
[0004] It is well known that a surface light source device of side
light type is applied to backlighting of a liquid crystal display.
The surface light source device supplies illuminating light from
the back face of a liquid crystal panel. This arrangement is
suitable for making the overall shape of the display thinner.
[0005] In general, a surface light source device of side light type
uses a rod-shaped light source, such as a cold cathode tube, as a
primary light source. The rod-shaped light source is provided on a
side of a guide plate (plate-like guide body). Illuminating light
emitted from the primary light source is guided inside the guide
plate through an incidence end face (hereinafter "incidence
surface") of the guide plate. Having been guided inside, the
illuminating light is propagated in the guide plate, whereby
illuminating light output is obtained from a major surface of the
guide plate.
[0006] A plate of generally uniform thickness and a plate of
gradient thickness are known as types of plate which can be used as
the guide plate. In general, the latter has higher illuminating
light output efficiency than the former.
[0007] FIG. 8 is an exploded perspective view of a surface light
source device of side light type using the latter type of guide
plate. FIG. 9 is a cross-sectional view taken along the line A-A of
FIG. 8. As shown in FIG. 8 and FIG. 9, the surface light source
device of side light type 1 comprises a guide plate 2, a primary
light source 3 which is provided along one side of the guide plate
2, a reflection sheet 4, a prism sheet 5 and a diffusion sheet
6.
[0008] The reflection sheet 4, the guide plate 2, the prism sheet 5
and the diffusion sheet 6 are laminatedly arranged. The guide plate
2 is a plate-like guide member which is bar-shaped in
cross-section. In this example, the guide plate 2 comprises a
scattering guide body. The scattering guide body comprises, for
instance, a matrix of PMMA (polymethyl-methacrylate) and a great
number of light-permeable particles which are uniformly dispersed
therein. The refractive index of these particles is different from
that of the matrix. Such a guide plate is called a scattering guide
plate.
[0009] The guide plate (light-scattering guide plate) 2 has major
surfaces providing an emission surface 2C and a back face 2B. The
emission surface 2C provides a prism surface as a light-controlling
surface. This prism surface comprises a great number of rows of
projections. As shown in partial enlargement at the section
indicated by reference symbol B, the projections, each having
slopes 2E and 2F, run almost perpendicular to the end surface
(incidence surface) 2A and are triangular in cross-section. As is
well known, this type of prism surface functions by gathering light
propagation direction toward the frontal direction in a surface
parallel to the incidence surface 2A.
[0010] A guide plate comprising transparent acrylic resin may, for
instance, be used instead of the scattering guide plate 2. When a
transparent guide plate is used, a diffusion surface is
conventionally provided on the back face 2B.
[0011] The primary light source 3 comprises a bar-shaped cold
cathode tube (flourescent lamp) 7 and a reflector 8, generally
semicircular in cross-section, which is provided to the back face
of the cold cathode tube 7. Illuminating light is supplied through
the opening of the reflector 8 toward the side end face of the
guide plate 2. A sheet-like regular reflection member comprising
metal foil or the like, or a sheet-like diffused reflection member
comprising white PET film or the like, is used as the reflection
sheet 4.
[0012] The prism sheet 5 is a light-permeable sheet-like member
comprising, for instance, polycarbonate. The prism sheet 5 is
normally provided with the prism surface facing the scattering
guide plate 2.
[0013] Each prism surface comprises a great number of projections
which run parallel to each other. As shown in partial enlargement
in the section indicated by reference symbol C, each projection has
a slope 5A and a slope 5B. Then, the prism sheet 5 is aligned so
that these projections run almost parallel to the end surface
(incidence surface) 2A. As is well known, the prism sheet 5 aligned
in this manner corrects the direction of light propagation to the
frontal direction in a surface which is perpendicular to the
incidence surface 2A.
[0014] The diffusion sheet 6, provided on the outer side of the
prism sheet 5, diffuses light propagation direction. In general,
the diffusion power of the diffusion sheet 6 is weak and
illuminating light is scattered weakly in order to prevent
interference fringes from occurring.
[0015] Illuminating light L from the primary light source 3 is led
through the incidence surface 2A into the guide plate 2. Inside the
guide plate 2, the illuminating light L propagates toward the end
while being repeatedly reflected between the back face 2B and the
emission surface 2C. During this process, the illuminating light L
is subjected to scattering action of the particles inside the guide
plate 2. A portion of light leaks from the back face, but the
mechanism of sheet 4 effectively returns this leaked light into the
guide plate 2. When the reflection sheet 4 comprises a diffused
reflection member, diffused reflection action also takes
effect.
[0016] As can be understood from FIG. 9, since the back face 2B is
inclined with respect to the emission surface 2C, the angle of
incidence of the illuminating light to the emission surface 2C
gradually decreases with each reflection of illuminating light L
from the slope 2B. This reduction in the angle of incidence
increases the incident components which are below the critical
angle to the emission surface 2C. This facilitates emission from
the emission surface 2C as the illuminating light nears the end.
Consequently, reduced brightness in regions which are far from the
primary light source 3 is prevented.
[0017] The illuminating light L output from the emission surface 2C
has properties of scattered light because it has experienced
scattering by light-permeable particles, or further diffused
reflection by the reflection sheet 4. However, as is well known,
the priority propagation direction (the main direction of
propagation) inclines in the end direction (opposite direction to
the primary light source 3) with respect to the frontal direction.
The prism sheet 5 corrects such directivity and corrects the
priority propagation direction to the frontal direction in a
surface perpendicular to the incidence surface 2A. The diffusion
sheet 6 weakly scatters the illuminating light, eliminating cause
of minute brightness inconsistencies such as interference
fringes.
[0018] In general, such a surface light source device 1 using the
bar-shaped guide plate 2 and the prism sheet 5 emits light in the
frontal direction more efficiently than a surface light source
device of the same type using a guide plate of generally even
thickness.
[0019] However, in the conventional device described above,
undesirable bright lines are generated along both side edges on the
emission surface 2C (left and right belt regions as viewed from the
primary light source 3). These bright lines tend to be especially
noticeable when the emission surface 2C comprises a light control
surface (a great number of projections) as described above. The
bright lines comprise localized fine belts of high brightness,
reducing the evenness of light output.
[0020] Such a tendency of bright lines might be lessened by using a
flat emission surface having no projection, but further restriction
is desirable. Furthermore, the guide plate 2 would lose its
function of correcting directivity in a surface parallel to the
incidence surface 2A. Cause of bright lines, which occur along both
side edges on the emission surface 2C, is thought to be that
illuminating light illuminates these side edges, undergoes one or
more internal reflections and is emitted locally from the emission
surface 2C.
[0021] The projections on the emission surface 2C relax the
critical angle conditions for light escape and consequently
facilitate local emission. Such a phenomenon is known as "(side
edge) over-reflection". There is a demand to remove such side edge
over-reflection in order to improve the quality of light output in
a surface light source device of side light type.
OBJECT AND SUMMARY OF INVENTION
[0022] The present invention aims to solve the abovementioned
problems of the conventional surface light source device ot side
light type. It is an object of the present invention to prevent
generation of bright lines along side edges of the emission surface
in a surface light source device of side light type. Described from
another point of view, the present invention aims to enable
generation of bright lines along side edges of the emission surface
to be prevented even when the surface light source device of said
type employs a guide plate having an emission surface which
comprises a light control surface.
[0023] The present invention is applied to a surface light source
device of side light type comprising a primary light source and a
guide plate, having an emission surface and a back face as major
surfaces thereof, wherein light is supplied from an incidence end
surface of the guide plate, and an edge is formed by a side face,
adjoining the incidence end surface, meeting with a back face.
[0024] In compliance with the features of the present invention,
multiple projections are provided on the back face in a region
along the edge. These projections run generally parallel to the
edge. A great number of projections, running generally
perpendicular to the incidence end surface, may be provided on the
emission surface.
[0025] In compliance with the preferred embodiment, projections
provided on the back face decrease in height as their distance from
the edge increases. Furthermore, in compliance with another
preferred embodiment, the projections, provided on the back face,
decrease in sharpness as their distance from the edge
increases.
[0026] The projections, provided on the back face of the guide
plate in a region along the edge, diversify and spread the
propagation direction of illuminating light, arriving from the
emission surface of the guide plate and the side face edge, and
light arriving from a frame, which forms a peripheral member.
Consequently, light arising from such illumination of the edge or
the frame is prevented from being strongly emitted locally from the
emission surface thereafter. As a result, generation of bright
lines along the side edges of the emission surface is prevented.
Bright lines of this type are especially liable to occur when a
prism surface is provided on the emission surface, but they can be
effectively controlled by the application of the present
invention.
BRIEF DESCRIPTION OF DRAWING
[0027] FIG. 1 (A) is a partially sectional view of a scattering
guide plate used in an embodiment of the present invention, and
FIG. 1 (B), a plan view of the same;
[0028] FIG. 2 is a partially sectional view illustrating a
mechanism of generating bright lines around a side face in order to
understand features of the present invention;
[0029] FIG. 3 is a partially sectional view illustrating emission
intensity around a side face in order to understand features of the
present invention;
[0030] FIG. 4 is a partially sectional view illustrating a function
for preventing bright lines of the scattering guide plate shown in
FIG. 1;
[0031] FIG. 5 is a diagram illustrating a function for preventing
over-reflection of the scattering guide plate shown in FIG. 1;
[0032] FIG. 6 is a partially sectional view of a scattering guide
plate used in another embodiment of the present invention;
[0033] FIG. 7 is a partially sectional view of a scattering guide
plate used in yet another embodiment of the present invention;
[0034] FIG. 8 is an exploded perspective view of a general
arrangement of a surface light source device of side light type;
and
[0035] FIG. 9 is a cross-sectional view taken along line A - A of
FIG. 8.
EMBODIMENT
(1) Embodiment
[0036] FIG. 1 illustrates features of a scattering guide plate 12
used in the present invention, showing a plan view (A) of under
surface 12B and a side view (B) from an incidence surface
(incidence end surface) 12A. The surface light source device of
side light type according to the present invention replaces the
guide plate 2 of the device shown in FIG. 8 and FIG. 9 with a new
scattering guide plate 12.
[0037] The following explanation centers on structure and effect of
the scattering guide plate 12, with reference to FIG. 8 and FIG. 9
as necessary. In FIG. 8 and Fig. 9, reference symbols relating to
the scattering guide plate 12 are indicated in brackets.
[0038] As shown in FIG. 1, the scattering guide plate 11
(hereinafter, guide plate 12), which is generally wedge-shaped,
preferably comprise the same type of material as the scattering
guide body 2 (see FIG. 8 and FIG. 9). The matrix comprises, for
instance, polymethyl-methacrylate (PMMA) and a great number of
light-permeable particles, which have a different refractive index
to the matrix, are uniformly dispersed therein.
[0039] Light supplied from the primary light source 3 passes
through a thick end portion (incidence surface) 12A. The guide
plate 12 comprises major surfaces providing an emission surface 12C
and a back face 12B. The emission surface 12C provides a prism
surface as a light control surface. This prism surface comprises a
great number of rows of projections.
[0040] In FIG. 1, reference symbols EL and EU represent edges
corresponding respectively to the line of intersection between the
long triangular side face and the back face 12B, and the line of
intersection between the same side face and the emission surface
12C. The projections, which have slopes 12E and 12F, run generally
perpendicular to the incidence surface 12A and are triangular in
cross-section. As in the case of the guide plate 2, this prism
surface serves to gather the directions of light propagation to the
frontal direction in a surface parallel to the incidence surface
12A.
[0041] In compliance with the features of the present invention,
the back face 12B of the guide plate 12 comprises multiple
projections provided in a belt region ARI along the edge EL. These
projections run generally parallel to the edge EL and each has a
pair of slopes 12G and 12H. The projections are triangular when
viewed in cross-section. The height of the projections (that is,
depth of the troughs) preferably decrease as distance the edge EL
increases. This prevents sharp changes in properties at the
boundary between the belt regions ARI and the other region (that
is, the region including the center portion). As a result, the
boundary is less easily observed, thereby preventing deterioration
in the quality of illuminating light.
[0042] The range (width) of the belt regions AR1 preferably be set
so as to include all points (on the back face 12B) which satisfy
escape conditions relating to the critical angle in a case when
illuminating light L, illuminated onto the upper and lower edges EL
and EU and from there internally reflected in the back face 12B, is
thereafter internally reflected to the emission surface 12C
(assuming a case where the back face 12B was a flat surface).
[0043] The angle of the slopes 12G and 12H of the projections is,
for instance, 100 degrees. The repeat pitch (direction parallel to
the incidence surface 12A) is, for instance, generally 50 gm.
[0044] Having replaced the guide plate 2 of FIG. 8 and FIG. 9 with
the guide plate 12 explained above, the behaviour of light in the
present embodiment is given below.
[0045] As shown in FIG. 1, FIG. 8 and FIG. 9, illuminating light L
supplied from a fluorescent lamp 7 passes through the incidence
surface 12A and is led into the guide plate 12. Inside the guide
plate 12, the illuminating light L propagates toward the end while
being repeatedly reflected between the back face 12B and the
emission surface 12C. During this time, the illuminating light L
receives scattering action of the particles inside the guide plate
12. A portion of light leaks from the back face, but the reflection
sheet 4 effectively returns this leaked light into the guide plate
12.
[0046] Since the back face 12B inclines with respect to the
emission surface 12C, incidence angle of the illuminating light L
to the emission surface 12C gradually decreases each time the
illuminating light L is reflected off the back face 12B. This
decrease in incidence angle increases the amount of incident
components which are below the critical angle with the emission
surface 12C, thereby facilitating emission. Consequently, reduction
of brightness in regions far from the primary light source 3 is
prevented.
[0047] Illuminating light output from the emission surface 12C has
properties of scattered light because it has experienced scattering
by light-permeable particles, or further diffused reflection by the
reflection sheet 4. However, the priority propagation direction
inclines toward the end direction with respect to the frontal
direction. The prism sheet 5 corrects such directivity and revises
the priority propagation direction to the frontal direction in a
surface perpendicular to the incidence surface 12A. The diffusion
sheet 6 weakly scatters the illuminating light, eliminating cause
of minute brightness inconsistencies such as interference fringes.
Correction of directivity in a surface parallel to the incidence
surface 12A is achieved by the projections on the emission surface
(light control surface) 12C.
[0048] Light behaviour so far explained is basically the same as in
the conventional device of same type. The present invention differs
from the conventional technology by providing projections in the
belt region AR1. These projections influence the behaviour of light
bearing on the edges EL and EU, preventing the generation of the
above bright lines on the emission surface 12C.
[0049] After propagating through the guide plate 12, a portion of
the illuminating light escapes to the outside of the guide plate 12
from the sides (between edges EL and EU) and their vicinity.
[0050] A peripheral member (not shown in the diagram), such as a
frame member or a housing member, is provided around the guide
plate 12 to hold all the elements contained therein. The escaping
light illuminates the peripheral member, is reflected off it, and
next brightly illuminates the edges EL and EU. Bright lines are
generated when light resulting from these illuminations is locally
output from the emission surface 12C. Such local output from the
emission surface 12C arises after the illuminating light has
undergone a number of repeated reflections between the emission
surface 12C and the back face 12B.
[0051] FIG. 2 illustrates the behaviour of light after illumination
of the upper edge EU. Light from the edge EU propagates through the
inside of the guide plate 12, is reflected off the back face and
internally enters the emission surface. Let us assume that
projections are not formed in the region AR1 (equivalent to the
conventional technology) and that the back face is flat. If the
emission surface is flat (indicated by a dash and dotted line),
angle .theta. remains unchanged and becomes the incident angle to
the emission surface. Therefore, as long as the angle .theta. does
not drop below the critical angle (e.g. generally 43 degrees in the
case of a PMNA matrix), no local emission occurs and total
reflection is achieved (broken line).
[0052] However, when the emission surface comprises a prism surface
(solid line), the angle of internal incidence to the prism surface
is less than 0. As a result, conditions for total reflection are
easily broken, causing local emission as indicated by a solid
line.
[0053] FIG. 3 shows results obtained after further examining the
behaviour of illuminating light arising from illumination of the
frame, and investigating variations of emission according to
distance from the side face of the edge frame. Note that, as above,
FIG. 3 assumes that no projections are formed on the back face.
[0054] In FIG. 3, a graph (B) illustrating intensity variation is
depicted according to the same partial cross-section (A) as FIG. 2.
As shown in FIG. 3, it was discovered that a range AR3, which is
defined by the critical angle of internal incidence to the emission
surface (dependent on the relation of air to the refractive index
of the guide plate), exhibits greater emission intensity than an
inner side region AR4; in addition, bright lines accumulate more in
the range AR3 than in the region AR4, lowering the quality of the
illuminating light output.
[0055] Increased emission intensity in this side face region AR3 is
mainly due to illumination of light from the frame. It was also
learned that this increase of intensity becomes more noticeable
when a prism surface is formed on the emission surface 12C. Note
that, for diagrammatic purposes, in FIG. 2, FIG. 3, and FIG. 4 and
FIG. 5 described later, the critical angle is depicted as greater
than the angle represented by the solid line.
[0056] In the present embodiment, projections are provided in
region ARI to deter such tendencies and consequently prevent bright
lines. In FIG. 4, which explains this action, projections, formed
along the side face in region AR1 of the back face 12B, are
represented by a solid line. In contrast, the dash and dotted line
represents the back face in a case when it is assumed that no
projections are formed, and the back face is therefore flat (as in
the conventional technology).
[0057] If the back face 12B were flat, many of the components of
light arriving from the edge EU would be reflected off the back
face 12B and the reflection sheet 4 and would proceed toward the
emission surface 12C. This light is easily emitted from the
emission surface 12C, comprising a prism surface. However, when
projections (slopes 12G and 12H) exist as in the present
embodiment, light path changes.
[0058] That is, many of light components arriving from the edge EU
pass through the back face 12B ind reflect off the reflection sheet
4. From the back face 12B, the reflected light then proceeds once
again through the guide plate 12. Light path during this process is
bent due to refraction action of the slope 12H. In fact, there is a
spread in the path of illuminating light arriving from the edge EU.
When several light paths were examined, it was found that diffusion
into a great number of directions occurred in accordance with
incidence angle to the slopes 12G and 12H of the back face 12B. If
the back face 12B were flat, we could not expect such diffusion
into a great number of directions.
[0059] Utilizing this type of diffusion effect, it is possible to
avoid local emission from the emission surface 12C of light
illuminating the upper edge EU. As a result, the possibility of
bright lines being observed is noticeably reduced. According to the
results of these tests, bright lines arising from the upper edge EU
were restricted to the point where they were barely observed.
[0060] Next, FIG. 5 is a diagram to explain this function for
preventing quality reduction of light caused by frame-irradiation.
In FIG. 5, light illuminating the frame thereafter passes between
the edges EL and EU and through the side face, reaching the back
face 12B.
[0061] If the back face 12B were flat, many components of light
would be reflected off the back face 12B and the reflection sheet 4
and would proceed toward the emission surface 12C. This light is
easily emitted from the emission surface 12C, comprising a prism
surface. However, when projections (slopes 12G and 12H) exist, as
in the present embodiment, light path changes.
[0062] That is, many of light components arriving from the edge EU
pass through the back face 12B and reflect off the reflection sheet
4. From the back face 12B, the reflected light then proceeds once
again through the guide plate 12. Light path during this process is
bent due to refraction action of the slope 12H. As is the case with
illuminating light arriving from the edge EU, there is a spread in
the arrival path of light from the frame.
[0063] Therefore, as in light arriving from the edge EU, we can
expect diffusion into a great number of directions to occur in
accordance with incidence angle to the slopes 12G and 12H of the
back face 12B. If the back face 12B were flat, we could not expect
such diffusion into a great number of directions.
[0064] Further, since the height of the projections decreases
according to distance from the side face, the above light diffusion
function tends to be weaken as distance from the side face
increases. This matches the general tendency of bright lines being
less likely to be generated as distance from the side tace
increases.
[0065] Furthermore, (i) components of illuminating light which
arrive at the emission surface 12C directly from the lower edge EL
and (ii) light, illuminating the frame, which is not reflected from
the back face 12B and consequently arrives directly at the emission
surface 12C are both caused by a reduction in the slopes 12G and
12H formed on the back face 12B. These can be observed as increases
in bright lines and emission intensity. However, overall, the
action described above noticeably prevents generation of bright
lines.
(2) Modifications
[0066] The scope of the present invention is not limited to the
embodiment described above. For instance, the following
modifications are also permissible.
[0067] (a) The gradient of light diffusion function of the
projections provided in region ARI may be set in a manner other
than changing the height of the projections. As shown in FIG. 6,
for instance, the inclination of the slopes may be gradually
reduced. In this case also, the boundary between the inside and
outside of the region ARI is, of course, obscured.
[0068] (b) In the above embodiment, the pairs of slopes 12G and 12H
of the projections provided in the region AR1 were directly
connected, so that the projections were triangular when viewed in
cross-section. However, the present invention is not limited to
this. For instance, the pairs of slopes may be connected via a
circular arc. Or, as shown in FIG. 7, the projections may be made
circular-arc-shaped in cross-section while consecutively changing
each pair of slopes.
[0069] (c) In the above embodiment, the angle formed by a pair of
slopes (top angle) was 100 degrees. However, this is merely one
example of projection sharpness. Generally, the sharpness of the
projections may be set according to design in compliance with
conditions such as thickness of the guide plate.
[0070] (d) In the above embodiment, the projections were depicted
at a pitch of 50 .mu.pm. However, this is merely one example.
Generally, the pitch of the projections can be set according to
design.
[0071] (e) In the above embodiment, the diffusion sheet 6 was
provided on the outer surface of the prism sheet 5. However, where
necessary, the diffusion sheet 6 may for instance be provided to
the inner surface of the prism sheet 5. A diffusion sheet is
omitted on a case-by-case basis.
[0072] (f) Generally, a non-flexible prism sheet, or a plate-like
light control member (namely, a prism body) may be used instead of
a flexible prism sheet.
[0073] (g) In the above embodiment, a light-scattering guide plate
was used as the guide plate. However, light-scattering guide plate
can be replaced with a light-permeable guide plate. Furthermore,
there is no limit on the material or manufacturing method of
light-scattering guide plate.
[0074] (h) The cross-sectional shape of the guide plate does not
have to be wedge-shaped. For instance, a guide plate of even
thickness may be used.
[0075] (i) Two or more end surfaces of the guide plate may be used
as incidence surfaces. Multiple primary light sources may be
provided according to therewith.
[0076] (j) The above embodiment used a guide plate having a prism
surface provided on the emission surface thereof However, the
present invention is not limited to this. The present invention can
also be applied when using a guide plate having a flat emission
surface or a mat surface.
[0077] (k) The surface light source device of the present invention
may be applied for uses other than backlighting in a liquid crystal
display. For instance, the present invention may be widely applied
to various illuminating equipment and displays.
* * * * *